This chapter describes the currently available methodologies and their interpretation for susceptibility testing of anaerobes. At present, alternative testing methods include limited agar dilution, broth microdilution (for the B. fragilis group), and the Etest gradient strip method. Tigecycline, a glycylcycline derivative of minocycline, was recently introduced for treatment of intra-abdominal and skin and soft tissue infections. Several studies have found very good activity against anaerobes, with rare strains exhibiting tigecycline MICs of >8 µg/ml. Extensive taxonomic changes have occurred within the non-spore-forming gram-positive bacilli, especially with those previously in the genus Eubacterium. It is important to compare the drug-containing plates to the drug-free control plate when reading the tests, as different species of anaerobic bacteria can have very differently appearing spots, ranging from mucoid-opaque, as with the B. fragilis group, to gray-transparent, as with Bacteroides ureolyticus. Broth microdilution MIC determinations require criteria similar to those for the agar dilution procedure for reading end points: the concentration at which the most significant reduction in growth is observed and interpreted as the MIC. Current methodologies allow for accurate surveillance or individual isolate testing by most laboratories. Future studies comparing broth microdilution to the reference agar method and the anticipated development of an improved microdilution system will result in better standardization of the more-user-friendly method and, possibly, more widespread commercial availability.

b >Members of the B. fragilis group are presumed to be resistant. Other gramnegative anaerobes may be screened for β-lactamase activity by use of a chromogenic cephalosporins test if penicillin therapy is contemplated. Higher blood levels are achievable; infection with non-β-lactamase-producing organisms with higher MICs might be treatable.

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TABLE 3

b >Members of the B. fragilis group are presumed to be resistant. Other gramnegative anaerobes may be screened for β-lactamase activity by use of a chromogenic cephalosporins test if penicillin therapy is contemplated. Higher blood levels are achievable; infection with non-β-lactamase-producing organisms with higher MICs might be treatable.